While being conveyed through a communications network, transmitted data is subjected to various disturbances that may give rise to an error rate which is incompatible with the required quality of transmission. In order to mitigate this phenomenon, a transmission protocol is used based, in most cases, on splitting up the transmitted data into fragments, in detecting when a received fragment includes errors, and in requesting retransmission thereof.
In the open system interconnection (OSI) standard, which defines seven layers for data interchange between systems, sublayer 2.1 relates to error detection, and it serves to split up and reassembled data in frames, and to detect errors in frames. Existing methods and apparatuses which process sublayer 2.1 in the HDLC protocol, for example, are adapted to processing a continuous flow of data which appears at an input; they therefore make use of a method of splitting up data which is specific thereto: delimiting frames by adding a known pattern to the beginning and the end of each frame, which pattern is "01111110" for HDLC; and in systematically injecting a "0" each time five consecutive "1s" are recognized within a frame so as to avoid imitating frame-delimiting patterns.
These existing methods and apparatuses are not well adapted to asychronous time division because of the following features specific thereto:
the data is naturally split up in the form of cells comprising 36 bytes, for example (of which 32 constitute useful data), with the cells being delimited in known manner; and
after passing through a network, cells belonging to a large number of different calls are interlaced. Using existing apparatuses would therefore require data to be split up in redundant manner and would also require data to be de-interlaced prior to processing.
The object of the invention is to mitigate the drawbacks of these prior solutions and to enable high transfer rates to be used with asynchronous time division signalling messages by enabling the protocol of the layer 2.1 to operate using a minimum number of bytes in a cell.